A very big advantage of the spinner anemometer is that it has self alligning effect which means that it needs no adjustment to measure the correct wind direction. This is due to the rotation of the sonic sensors. Each sensor measures the wind from different positions during a rotation and the only way that the rotor can be alligned directly into the wind, is when the sensors measure the same wind from different positions on the rotation. This self alligning effect is very important because allignment of instruments on wind turbines is otherwise a very difficult task often overlooked.
The rotation of the spinner anemometer has the natural consequence that the average values of each sonic sensor, over time, must be the same. This feature can be used to make an "internal" calibration of the sensors against each other. This internal calibration will compensate for inaccuracies of the spinner shape, off-axis mounting and of inaccuracies of wind sensor mountings. The internal calibration assures that instantaneous wind measurements can be made very accurate, which is very important for turbulence measurements.
Another consequence of the rotation of the spinner anemometer is that each wind sensor can be used separately to give average wind speed and wind direction values by averaging over time. This means that a single sonic sensor will be enough to measure all wind speeds and makes the system accurate, redundant and very robust. For more about the single sonic sensor versus three sensors, read the paper: "Operational experiences with a spinner anemometer on a MW size wind turbine".